Cosmetics are assumed to be used for a long term, and constantly at a risk of contamination. Therefore, a preservative is indispensible. On the other hand, since the image of preservatives is worsening due to the aspiration to safety in recent years, various preservatives such as paraben and the like are not favored by consumers in the market. Accordingly, a technique for preserving cosmetics without using such preservatives is desired (non-patent document 1).
However, many problems occur in preserving cosmetics without adding a preservative, and many problems were experienced in actual formulation design, such as (i) limitation on pH (need to lower the formulation pH and the like), (ii) limitation on the containers and methods to be used due to an insufficient antiseptic effect, and (iii) an adverse influence on the sense of use by a material added for an antiseptic effect (e.g., to decrease water activity and the like) since it causes sliminess and stickiness (non-patent document 2). In such cases, prevention of mold (fungus) is known to be particularly difficult since it shows a wider water activity permitting growth, and a growth optimal pH on the acidic side (non-patent documents 1, 2).
In addition, since cosmetics are often used around water, which is particularly suitable for the growth of microorganisms, and refillable products are increasingly used backed by the awareness of environmental issues in recent years, the risk of contamination with microorganisms increases due to the reuse of containers. In this situation, the standard shown in, for example, the Pharmacopoeia (the Japanese Pharmacopoeia, supplement of the 14th edition, 5. preservatives effectiveness test) may not be sufficient. Generally, it is requested that three kinds of bacteria and Candida reach an eradication level of 10 or less viable counts within 7 days from inoculation and do not grow thereafter, and Aspergillus brasiliensis show a remarkable decrease in the viable counts at an early stage after inoculation. Particularly, when a container is reused, bacteria that acquired resistance to antibacterial agents may emerge, and the standard for bacteria is set to a higher level than usual, as evidenced by the use of a test method for examining the easiness of developing resistant bacteria and the like.
While a method of preserving cosmetics by using alkylhydroxamic acid and alcohol, particularly diols, has been proposed, this method shows only an insufficient antiseptic effect on Aspergillus brasiliensis and the like (patent document 1). Furthermore, since low molecule diols show a high influence on the sense of use and formulation, an increase in the amount of use thereof is not preferable.
Since 4-hydroxyacetophenone has an antiseptic effect, a method of preserving cosmetics by using same in combination with various antiseptic materials has been proposed (patent documents 2, 3). However, application thereof to cosmetics is problematic, since 4-hydroxyacetophenone has a peculiar odor unfavorable for cosmetics, and cannot be added in large amounts to enhance an antiseptic effect and the like.
Furthermore, hexylglycerol is also known to have an antiseptic effect. However, addition thereof in large amounts to cosmetics and the like affects texture and viscosity, and is feared to cause irritation. Therefore, the amount of use thereof is limited. On the other hand, low-irritative cyclohexylglycerol is known to show only a weak antiseptic effect and requires addition in large amounts, thereby causing an influence on the texture and viscosity.
It has been reported that decanoylproline or a salt thereof has a superior moisturizing effect, and can provide cosmetics superior in the sense of use while suppressing the smell of antimicrobial agents having a problematic smell (patent document 4).